Did you make this instructable?

If you talking about the big slabs you can get always keep it fully wrapped up in the plastic it comes wrapped in and store somewhere nice and cold and dark. If your not going to be using it for a long time ever week or so unwrap it and spray it with water all over but don't swamp it with water. If you talking about keeping something you've made out of clay dry it all depends on the thickness of the clay to how long it will take to fully dry. I would spray it with water at least twice a day to keep it wet but again, don't swamp it with water!

If you're buying it, the manufacturer or the package will probably tell you the answers to both of your questions. Sorry I can't be clear but there are just so many types and brands of clays out there. Typically to slow down the process, you add water. Usually it's enough to just moisten your hands a little while you work.